(1) Field of the Invention
The present invention relates to an engine structure, in particular relating to a drive mechanism for a four-cycle engine mounted on a snow vehicle.
(2) Description of the Prior Art
Conventionally, most small snow vehicles such as snowmobiles and the like use two-cycle engines, which are relatively simple in structure, light and compact and yet powerful. Recently, however, because of regulation of exhaust gas due to environmental issues or aiming at improvement of fuel consumption, there is a trend toward employing four-cycle engines.
In contrast to two-cycle engines, which are compact, light and high in power, four-cycle engines need a camshaft and oil lubrication, inevitably tending towards large size.
Further, when a high-speed type four-cycle engine is used to secure the engine power, driven force should be transmitted using a reduction shaft, which brings with it a space problem for the disposal of the reduction shaft.
In general, when an engine is mounted on a snowmobile, it is preferred that the engine should be set as low as the steering tie-rod so that the center of gravity of the vehicle with the engine mounted is positioned at a low position to improve the maneuverability and that the engine should be set as close to the steering post as possible so that the center of gravity will be set at the center of the body.
However, when a high speed type four cycle engine is mounted for a snowmobile, an engine 202 needs to be arranged in an engine room 30 located on the front side of a snow vehicle (snowmobile) 201 and a reduction shaft, oil pump, water pump and others should be arranged on the rear side of the engine 202 (on the intake side), as shown in FIG. 1. Accordingly, if the center of gravity of the engine is attempted to be set at the center and at the lowest possible position in order to improve the maneuverability, there occurs the problem that a crankcase 205 including the reduction shaft interferes with a steering post 225 or heat exchanger 226 while an oil pan 209 and/or crankcase 205 interfere with the body bottom or a frame 10. In the drawing, 11, 13 and 28 designate a front frame, steerable ski-runner and windshield, respectively.
To solve the above problem, engine 202 should be arranged at the rear and at an upper position, in engine room 30 located on the front side of snow vehicle 201. However, since this conventional engine arrangement causes the cylinder head, designated at 204, to interfere with an engine hood 29 or headlight 31, the engine hood 29 needs to be enlarged, which produces new problems, i.e., increase in weight and degradation of maneuverability.